1. Field of Invention
The present invention relates to semiconductor memory device. More particularly, the present invention relates to nonvolatile memory device with selection gate and fabrication method.
2. Description of Related Art
Flash memory device allows multiple-time erase and program operation inside system. As a result, flash memory is suitable to many of advance hand-held digital apparatuses, such as solid state disks, cellar phones, digital cameras, digital movie cameras, digital voice recorders, and personal digital assistant (PDA), that are demanding a low-cost, high-density, low-power-consumption, highly reliable file memory.
Basically, data flash memory has two typical cell structures. One is double poly NAND type memory cell with poly1 as floating gate to store charges; and the other one is single poly SONOS cell with SiN as storage node. A conventional NAND flash includes numerous strings of series connected N-channel transistor. Device operation of NAND flash utilizes channel Fowler-Nordheim (FN) mechanism for programming and erasing, and cell size for the NAND type flash memory cell is around 4–5F2, here F represents a critical dimension used in semiconductor fabrication as a dimension reference for describing cell size.
On the other hand, conventional SONOS technology is a NOR type flash memory with buried N+ structure. FIG. 1 is a cross-sectional view, schematically illustrating a conventional SONOS flash memory. Device operation of SONOS cell is adopted channel hot carriers for programming and B—B hot holes for erase. FIG. 2 is top view, schematically illustrating the layout of the memory cell with respect to FIG. 1. In FIG. 1 and FIG. 2, the N-well 102 and the P-well 104 are formed in the substrate 100, such as a P-type substrate. Since the whole flash memory includes memory region and the logic region, the various wells are formed to have the CMOS device. The memory cells are formed in the T(triple)P-well 104 as can be understood by the ordinary skilled artisans. For this kind of flash memory, the bit lines BL0, BL1, . . . , BLm 106 are formed in the substrate with strip doped regions. This kind of design for the bit lines is also called the buried bit line design. FIG. 2 only shows the layout for the bit lines 106 and the word lines 110. The charge storage is achieved by the oxide 108a/nitride 108b/oxide 108c (O/N/O) structure layer 108. The word line 110 also serves as the necessary gate.
The operation mechanisms for above cell design in programming, reading and erasing operations are shown in FIG. 3. The word line (WL) is also the gate electrode. The adjacent two bit lines serve as the source/drain (S/D) region in the substrate. The oxide/nitride/oxide (O/N/O) structured layer is between the gate electrode and the substrate, in which the nitride layer is used to store the charges. Due to the charges in the nitride layer almost not moving, the injected charges can be localized in the nitride layer. Therefore, according to the voltages applied on the bit lines, for example for the programming operation at the top two drawings. For the operation shown in left drawing, due to the hot electrons, desired charges are stored in the nitride layer, in which the charges are localized at the one side. However, for the reversed direction shown the right drawing, the charges are stored in the nitride at the left side. Then, for the reading operation, according to the reading direction, the two sides can be separated read. The stored charges change the threshold voltage, so that the stored binary data can be sensed. The erasing operation is to inject the band-to-band (B—B) holes to the nitride layer to neutralize the electrons, so as to erase. Basically, The programming operation is to change the threshold voltage from low to high, and the erasing operation is to change the threshold voltage from high back to low. The operation should be well known by the skilled artisans and the detailed description is skipped.
However, the conventional SONOS flash memory has the disadvantages. As shown in FIG. 3, charges in nitride layer may laterally diffuse between twin bits in SONOS cell. Although the charge trapped in nitride or other discrete comparing to charges in conductor film, the charges still drift at high temperature for a long period, indicating charge loss. When storage charges drift occurs, the charge can move to the location where the erased hot holes can not reach and result in un-erased bit or the bit can not be ignored during reverse read, i.e. so called second bit effect. Further, due to the unlimited storage region caused by the nitride on the whole channel, the program and erase region can not overlap properly and also results in un-erased bit or second bit effect. In addition, the conventional hot carriers with low injection efficiency for programming consumes a larger current that can't support page mode programming.
To improve the above disadvantages, another conventional structure of memory device, a charge storage O/N/O structure arranged between the buried bit line and a selection gate is proposed. The selection gate is located between the bit lines to provide the limited storage region, so the second bit effect described above can be removed FIGS. 4A–4B are cross-sectional views, schematically illustrating another conventional structure of memory device. In FIG. 4A, for example, by applying the programming voltage on the bit line BL and the selection gate SG to create a source side hot electron injection with low current and high efficiency, the hot electrons can be driven into the nitride layer. Likewise, when the memory cell is intended to be erased, an erasing voltage is applied to drive holes into the nitride layer to neutralize the electron charges.
However, in this conventional structure as uniquely investigated by the invention, the erase operation cannot efficiently remove the trapped electrons in the nitride layer of the O/N/O structure. The mechanism is described in better detail as follows. Since the nitride layer is the dielectric and is not a conductive material, the trapped hot electrons during programming operation basically are localized near to the selection gate without mobility due to the source side hot electron injection. In other words, the electrons have the distribution as shown in FIG. 4C. However, when the erase operation is performed, the holes h+ have the tendency to be localized at the region near the junction of bit line BL, as shown in FIG. 4D. Due to low mobility for the carriers in the nitride layer, the electrons and the holes are not fully neutralized with respect to the position. This situation still exits even though the amounts of electrons and the holes are equal. Then, it would cause the read error due to the residual charges.
In another consideration, if the electrons are to be completely removed, then the amount of hole should increase to cover over the electron distribution. However, in this situation, after the erase operation, the residual positive holes stay in the nitride layer. It would cause the error for the next programming operation. In other words, the conventional structure in FIG. 4A has the issue in read error and programming error. If the density of the subsequent charges is increased each time to cover over the previous charges, then the residual charges are accumulated each operation. It then causes a failure of the memory cell. Also and, the operation voltages are not stable for each time of operations.
In another words, due to the different direction of charge transport for program and erase, it is hard to obtain the same electron or hole distribution in discrete storage layer (FIGS. 4C–4D), indicating low program and erase (Program/Erase) cycling window. This phenomenon can be explained as following: For erased cell, a lot of holes will trap on the storage layer near BL junction. When the erased cell is programmed again, however, electrons are difficult to inject to the storage layer over BL junction because the high electrical field distributes on the interface of select transistor and memory transistor. Therefore, in order to obtain reasonable program threshold voltage, it needs to inject more electrons than that needed in first program to compensate the effect of holes accumulating on unwanted location. After several P/E cycling, it may cause a peak of holes piling up on the storage layer near the BL junction. The unwanted holes may make the threshold voltage of programmed memory reduction, or cell not to be programmed again. Another impact of the pile-up of holes is to cause the potential failure of data retention. The reasons are described as follows. Although the storage layer like nitride is not conductive, the charges is still able to migrate in the nitride and the mobility of hole is higher than that of electrons. For a period of high temperature baking that is frequent used in reliability testing, the holes will migrate laterally. Especially, due to electrons and holes piled up on both ends of storage layer, the electrical field is created between them. The electrical field will accelerate the migration speed of holes. The migration of holes will decrease the threshold voltage of program or erase cell, indicating the poor retention performance. The case will be worse with the increase of P/E times.
On the other hand, such a memory is not allowed to over erase according to its array structure. To overcome this over-erase issue, the complicated erase verification is necessary. Besides, once the cell is over erased, the BL to BL leak will be serious to make the error bit. All of above discussed issues are at least to be solved, in order to improve the performance of programmable memory device. Particularly, the issues occurred in FIGS. 4C–4D, as investigated by the invention, are necessary to be solved.